MEMS device generally comprise a mechanical structure integrated heterogeneously on an electronic substrate containing wafer-level circuitry, where the mechanical structure is vacuum encapsulated on the wafer. FIG. 1A illustrates a known MEMS device 10 comprising an electromechanical structure 12 integrated heterogeneously on an electronic substrate 14 that comprises wafer-level circuitry (not shown). A cap wafer 16 having a cavity is attached to substrate 14, for example by a ring of welding 18, so as to vacuum-encapsulate the electromechanical structure 12 on the wafer. One way to form a thin MEMS device 10 comprises assembling device 10 using a device wafer 14 and a cap wafer 16 having first thicknesses, then thinning said wafers. However, the inventors have noted that if the substrate wafer 14 contains electronic circuitry (not shown), thinning the substrate wafer can cause the substrate to bow due to the high stresses in the top films used in typical IC processing. Depending on how rigid and thick the capping wafer is, such stress can lead to cracking of the capping wafer.
Additionally, the cap wafer is traditionally etched either with wet or dry processes to expose wire bonding pads 20 previously formed on the device wafer 14 and connected to the wafer-level circuitry (not shown). However, on one hand wet etching through the capping wafer 16 produces sloped sidewalls, which prevents making device 10 compact. On another hand, dry etching of the cap wafer 12, such as Bosch etching, allows obtaining side walls of the cap wafer that are substantially perpendicular to its top surface, but the polymer passivation step in the etch recipe that is needed to produce such substantially perpendicular walls can leave a Teflon-type polymer film on metal bond pads 20; thereby preventing subsequent wire bonding to these pads.
There exists a need for a method that allows high yield manufacturing of thin MEMS devices and/or MEMS devices with walls of the cap wafer that are substantially perpendicular to the top surface of the cap wafer